In global navigation satellite systems (GNSSs) such as e.g. GPS (Global Positioning System), the receivers detect their geographic position on the basis of received signals which have been transmitted by satellites forming part of a global satellite constellation. The satellites belonging to the GPS satellite constellation will emit their signals on a plurality of carrier frequencies. Each carrier is modulated by at least one pseudo-random binary code frequency PRN (pseudo random noise) which consists of a pseudo-random, periodically repeating sequence of zeros and ones, or of an aperiodic sequence of zeros and ones. The PRN sequences are also referred to as ranging codes because they make it possible to estimate the distances (“ranges”) between receiver and satellite. The PRN code sequences used are distinguished in that they have a clear “peak” in the autocorrelation function, which allows for a propagation time measurement.
Each satellite uses its own PRN code sequence, which is why the receiver can assign the received signal to that satellite which transmitted it. The receiver will compute the difference between the point of time that the satellite transmitted the signal—wherein this information is contained in the signal itself—and the point of time that the receiver itself received the signal. On the basis of this difference in time, the receiver will compute its own distance from the satellite. The receiver can compute its own global geographic position by reference to the obtained distances to at least four satellites.
For obtaining the temporal difference between said point of time that the signal is transmitted and said point of time that this signal is received, the receiver will synchronize a locally generated PRN reference code sequence with the PRN code sequence contained in the received signal. In this manner, the receiver will obtain the measure of the temporal deviation of the locally generated PRN reference code sequence in relation to the satellite time and will compute the distance. The synchronization operations include the acquisition of the PRN code sequence of the satellite and its tracking (code tracking). Further, in the receiver, the phase of the carrier which is used by the satellite for emitting the PRN code sequence and the navigation data will normally be tracked (phase tracking).
Presently, a new satellite navigation system bearing the name Galileo is being realized, which offers very high precision and various services. The development of the new Galileo satellite navigation system opens up the possibility of new applications, among them the so-called Safety-of-Life (SoL) services. These services require a special interference resistance with respect to multi-path scattering and interference. A technical solution to this resides in receivers with an antenna array, i.e. a multi-element antenna consisting of a plurality of individual antennas (antenna elements), and a correspondingly designed subsequent signal processing, e.g. adaptive beamforming for well-aimed interference suppression.
For achieving a reliable and precise signal processing, particularly for DOA (Direction of Arrival) estimation and adaptive beamforming (adaptive nulling), it is required that the analog reception signal paths—following the individual antennas of the antenna array—of a preprocessing unit of such a receiver of satellite navigation signals be precisely calibrated in phase and amplitude.
In WO-A-2010/136498, a method and a receiver for the receiving and processing of satellite navigation signals are described. In this known method and receiver, a calibration signal is generated which is configured like the satellite navigation signals, i.e. again comprises a PRN code sequence and a carrier frequency. A block diagram of the known satellite navigation signal receiver is shown in FIG. 1.
Said known receiver 10 comprises an antenna array 12 having a plurality of individual antennas 14 which are arranged in array form and whose antenna outputs 16 are connected to the inputs of (LNA—Low Noise Amplifier) amplifiers 18. The outputs 17 of these amplifiers are connected, via cables 20, to an analog preprocessing unit 22 comprising various signal processing units which in the present context shall not be explained more closely and which are described in greater detail in WO-A-2010/136498. The subject matter of WO-A-2010/136498 is hereby included, by way of reference, in the subject matter of the present application. The analog preprocessing unit 22 comprises a number of signal transmitting and processing channels 24 identical to the number of individual antennas 14. The signal inputs 26 of the analog preprocessing unit 22 are connected via the cables 20 to the outputs of the amplifiers 18. The analog preprocessing unit 22 itself likewise comprises signal outputs 28 which via cable connections 30 are connected to the inputs 32 of a digital signal processor 34. For each channel, the digital signal processor 34 comprises an analog/digital converter 36 and code acquisition as well as code and carrier-frequency tracking units 38 with correlation units 40 and PLL/FLL modules 42,44 which are used for detection of raw data for further processing in a signal processing unit 46. This processing technique is known per se and shall not be explained here in further detail.
In the known satellite navigation signal receiver 10, there is further generated a calibration signal which, like the satellite navigation signals, comprises a PRN code and a carrier frequency. This calibration signal is generated in a calibration signal generating unit 48. The digital signal processor 34 operates at an operating frequency which is generated by a reference frequency generating unit 50. This unit 50 also controls a PLL synthesizer 52 whose output signal is used at 54 for up-mixing the calibration signal so that the calibration signal will have a carrier frequency within the carrier frequency bands of the satellite navigation signals, and further for down-mixing the received satellite navigation signals in the analog preprocessing unit 22 (see at 56).
The reception signals of GNSS systems at the receiver are very weak. The reception power of these signals is in the range of merely a few femto-watts. Thus, however, also the calibration signal generated in the navigation receiver has to be quite weak because otherwise it would “cover” the navigation signals. The calibration signal and the received satellite navigation signals are very vulnerable to intended and unintended interference. However, exactly for DOA estimation and adaptive beamforming, it is desirable that the calibration of the satellite navigation signal receiver is robust toward interference signals. One could increase the robustness of the calibration signal e.g. by                using longer codes and correlation times,        increasing the bandwidth of the calibration signal, and/or        increasing the power of the calibration signal.        
All of this, however, can be realized only with additional expenditure and/or longer signal processing times (correlation times). Temporal variations of the calibration signal can be tracked only insufficiently. In case of intended interference, these measures are often useless.